In the art of lithographic printing plates, there are many options for producing an image useful for printing. These include an additive, one-step resin reinforced image; a subtractive photo-polymer image; a subtractive diazo-resin reinforced image; a subtractive photo-polymer on diazo image; an additive one-step reinforced image post-cured with heat, a subtractive diazo image; photo-polymer image reinforced and post-exposed, bi-metal and tri-metal plates and resin particles in a diazo matrix. The image is conventionally formed by exposing a plate coated with a light sensitive material to UV light through a full size film negative or positive.
The lithographic printing industry has been searching for a dependable laser imagable lithographic printing plate for more than ten years. In the newspaper industry the laser imagable plate would enable the newspaper to assemble its copy in a metropolitan center and to have it printed in a plant far removed from the center. In some instances, this would be done in a satellite printing plant situated only a few miles from the editorial offices, and in other instances, the printing plants would be located many thousands of miles away. Additionally, other printing establishments, among them small newspapers, have been interested in a laser imagable plate which would permit them to eliminate their silver halide negatives.
There exist three modes of direct laser imagable plates today: electrostatic, ultraviolet, and infrared.
The electrostatic plates involve the electrostatic toning of a lithographic substrate either directly or by transfer from an intermediate.
The ultraviolet imagable plates are imaged by very powerful, generally Argon-Ion lasers which require large inputs of energy as well as water cooling. Typical units are manufactured by EOCOM, a division of American Hoechst and are powered in some cases by lasers delivering as much as 18 watts of output energy. Since the Argon-Ion generates only about 15% of its energy in the UV wave lengths, it requires a very powerful laser. Efficiency in the 4-5 watt total spectrum range has been claimed, but present information indicates that there is no commercial unit performing at this energy level.
The infrared YAG laser manufactured by LogE, a division of Crosfield Data Systems, Inc. generates its energy at 10,600 Angstroms, i.e., one micron. It has been used in the past by placing a graphite impregnated mask over a bare lithographic plate and blasting the graphite and its resin binder onto the plate where it is subsequently fused and lacquered. The plate requires a graphite coated Mylar or PVC film mask (e.g. U.S. Pat. No. 3,964,389) and delivers a product which is variable in quality and expensive because of the need for both a plate and a coating mask.